Exhaust gas treatment method, exhaust gas treatment device, and carbon fiber manufacturing system

ABSTRACT

One object of the present invention is to provide an exhaust gas treatment method for treating exhaust gases discharged from a carbon fiber manufacturing steps which can suppress a cost increase due to an increase in an amount of an exhaust gas treated, the present invention provides an exhaust gas treatment method including: a first combusting step in which a carbonizing step-exhaust gas discharged from a carbonizing step in which the fibrous substance is carbonized in an inert gas atmosphere is treated; and a second combusting step in which a flameproofing step-exhaust gas discharged from a flameproofing step in which the fibrous substance is flameproofed in an air atmosphere and a first combusting step-exhaust gas discharged from the first combustion step are treated; and an air separating step in which nitrogen for producing the inert gas atmosphere in the carbonizing step, and the oxygen-enriched air used in the first combusting step are produced by separating air.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to an exhaust gas treatment method, anexhaust gas treatment device, and a carbon fiber manufacturing system.

RELATED ART

Carbon fibers have been used as a reinforcement for various materialsbecause they have superior specific strength, specific modulus, hetresistance, chemical resistance, and so on. In general, when carbonfibers are produced, the production method includes a plurality of stepsin order to obtain desired properties. For example, when acrylic fiberis used as a precursor of carbon fibers, flameproofing fiber is producedby pre-oxidizing in the air at 200 to 300° C. in a first step(flameproofing step). Then, carbon fibers are produced by carbonizing at300 to 2,000° C. in an inert atmosphere (carbonizing step). In addition,when carbon fibers having a high elastic modulus are produced, thecarbon fibers obtained in the carbonizing step is graphitized at 2,000to 3,000° C. in an inert atmosphere (graphitizing step).

However, during the flameproofing step, the carbonizing step, and thegraphitizing step, an exhaust gas is generated. Specifically, since thecarbonizing step, and the graphitizing step are carried out in an inertatmosphere, gas containing hydrogen cyanide, ammonia, carbon monoxide,carbon dioxide, methane, tar, etc. which are decomposed components ofthe flameproofing fiber, and the inert gas, such as nitrogen, as a basecomponent (below, exhaust gas generated in the carbonizing step and thegraphitizing step is denoted respectively by “carbonizing step-exhaustgas” and “graphitizing step-exhaust gas”) is generated (Patent DocumentNo. 3).

On the other hand, since the flameproofing step is carried out in theair, gas containing hydrogen cyanide, ammonia, carbon monoxide, carbondioxide, methane, tar, and so on which are decomposed components of theacrylic fiber, and oxygen, nitrogen, and argon, as base components(below, exhaust gas generated in the flameproofing step is denoted by“flameproofing step-exhaust gas”) is generated (Patent Documents Nos. 2and 4).

As explained above, the exhaust gas which is generated in theflameproofing step, the carbonizing step, and the graphitizing stepcontains strong harmful gas, such as hydrogen cyanide, ammonia, and soon. Accordingly, an exhaust gas treatment method for detoxifying theexhaust gas generated in these steps is required.

As a conventional exhaust gas treatment method, a method in which theflameproofing step-exhaust gas and the carbonizing step-exhaust gas areblown into one treatment furnace (combustion chamber), and decomposed byair combustion has been well-known (For example, Patent Document No. 1).As another conventional exhaust gas treatment method, a method in whichthe flameproofing step-exhaust gas and the carbonizing step-exhaust gasare decomposed by air combustion in a separate treatment furnace,respectively has been well-known (For example, Patent Document No. 2).

PRIOR ART DOCUMENTS Patent Documents

[Patent Document 1] Japanese Unexamined Patent Application, FirstPublication No. 2011-021779

[Patent Document 2] Japanese Unexamined Patent Application, FirstPublication No. 2001-324119

[Patent Document 3] Japanese Unexamined Patent Application, FirstPublication No. 2012-067419

[Patent Document 4] Japanese Unexamined Patent Application, FirstPublication No. 2003-113538

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

Incidentally, the flameproofing step-exhaust gas, and the carbonizingstep-exhaust gas and the graphitizing step-exhaust gas are differentfrom each other in the concentration of hydrogen cyanide, etc., and thecomposition of the base gas (existence of oxygen). Therefore, when theflameproofing step-exhaust gas, and the carbonizing step-exhaust gas andthe graphitizing step-exhaust gas are decomposed in one treatmentfurnace, there is a problem that hydrogen cyanide, ammonia, etc. cannotbe sufficiently decomposed, and a large amount of NO_(X) is generated bythe decomposition.

In addition, when the treatment capacity of the treatment deviceincreases due to an increase in size of the treatment device, it isnecessary to increase the amount of air or the like necessary for thecombustion treatment. Accordingly, in the conventional technology, it isnecessary to increase the capacity of the treatment gas dustcollector/exhaust blower.

Further, when oxygen-enriched air is used in place of air for thepurpose of increasing the treatment capacity, an oxygen supply source isnecessary. Accordingly, there is concern that the cost for treatmentwill increase. In order to suppress the cost increase, it is necessaryto optimize the exhaust gas treatment system as a whole.

In addition, when the flameproofing step-exhaust gas, the carbonizingstep-exhaust gas and the graphitizing step-exhaust gas are decomposed inseparate treatment furnaces, there is a problem that hydrogen cyanide,ammonia, and so on, can be sufficiently decomposed, but an amount offuel used for combustion increases. In addition, since the number oftreatment equipment increases, there is a problem that the equipmentcost and the maintenance cost increase.

In consideration of the above-described problems, an object of thepresent invention is to provide an exhaust gas treatment method whichprevents the generation of NO_(X), and treats the flameproofingstep-exhaust gas and the carbonizing step-exhaust gas with a smallamount of fuel, an exhaust gas treatment device which can treat theflameproofing step-exhaust gas and the carbonizing step-exhaust gas witha small amount of fuel, and an exhaust gas treatment system which cansuppress a cost increase due to an increase in the amount of gastreated.

Means for Solving the Problem

In order to solve the above problems, the present invention provides thefollowing exhaust gas treatment devices, exhaust gas treatment methods,a carbon fiber manufacturing equipment, and carbon fiber manufacturingsystems.

(1) An exhaust gas treatment device which is configured to treat exhaustgases discharged from steps of manufacturing carbon fiber from a fibroussubstance,

wherein the exhaust gas treatment device includes:

a first combustion furnace which is configured to combust a carbonizingstep-exhaust gas discharged from a carbonizing step in which the fibroussubstance is carbonized in an inert gas atmosphere; and

a second combustion furnace which is provided at a post-stage of thefirst combustion furnace, and configured to combust a flameproofingstep-exhaust gas discharged from a flameproofing step in which thefibrous substance is flameproofed in an air atmosphere and a firstcombusting step-exhaust gas discharged from the first combustionfurnace;

the first combustion furnace and the second combustion furnacecommunicate through a throttle portion;

the first combustion furnace is provided with a first burner which isconfigured to combust a carbonizing step-exhaust gas by a fuel and anoxygen-enriched air; and

an air separation device which is configured to produce nitrogen forproducing the inert gas atmosphere in the carbonizing step and theoxygen-enriched air supplied into the first burner.

(2) The exhaust gas treatment device according to (1), wherein theexhaust gas treatment device further includes a second burner which isconfigured to supply a fuel into the second combustion furnace.(3) An exhaust gas treatment method for treating exhaust gasesdischarged from steps of manufacturing carbon fiber from a fibroussubstance,

wherein the exhaust gas treatment method includes:

a first combusting step in which a carbonizing step-exhaust gasdischarged from a carbonizing step in which the fibrous substance iscarbonized in an inert gas atmosphere is treated; and

a second combusting step in which a flameproofing step-exhaust gasdischarged from a flameproofing step in which the fibrous substance isflameproofed in an air atmosphere and a first combusting step-exhaustgas discharged from the first combustion step are treated;

in the first combusting step, the carbonizing step-exhaust gas iscombusted at a low oxygen ratio of 0.8 or less by a fuel and anoxygen-enriched air;

in the second combusting step, the first combusting step-exhaust gas andthe flameproofing step-exhaust gas are combusted by using heat of thefirst combusting step-exhaust gas; and

nitrogen for producing the inert gas atmosphere in the carbonizing step,and the oxygen-enriched air used in the first combusting step areproduced by separating air.

(4) The exhaust gas treatment method according to (3), wherein thecombustion of the first combusting step-exhaust gas and theflameproofing step-exhaust gas is promoted by supplying a fuel in thesecond combusting step.(5) A carbon fiber manufacturing equipment including equipment which isconfigured to produce carbon fiber from a fibrous substance, and exhaustgas treatment equipment which is configured to treat exhaust gases fromthe equipment which is configured to produce carbon fiber from a fibroussubstance,

wherein the carbon fiber manufacturing equipment includes:

a flameproofing furnace which is configured to pre-oxidize andflameproof the fibrous substance in an air atmosphere;

a carbonizing furnace which is provided at a post-stage of theflameproofing furnace and configured to carbonize the fibrous substanceafter flameproofing in an inert atmosphere;

a first combustion furnace which is configured to combust a carbonizingstep-exhaust gas discharged from the carbonizing furnace;

a second combustion furnace which is configured to communicate with thefirst combustion furnace through a throttle portion, and combust a firstcombusting step-exhaust gas and a flameproofing step-exhaust gasdischarged from the flameproofing furnace;

a first burner which is provided in the first combustion furnace so asto combust the carbonizing step-exhaust gas by a fuel and anoxygen-enriched air; and

an air separation device which is configured to produce nitrogen forproducing the inert gas atmosphere in the carbonizing furnace and theoxygen-enriched air supplied into the first burner from air.

(6) A carbon fiber manufacturing system including a step in which carbonfiber is produced from a fibrous substance and a step in which exhaustgases discharged from the step in which carbon fiber is produced from afibrous substance are treated,

wherein the carbon fiber manufacturing system includes:

a flameproofing step in which the fibrous substance is pre-oxidized andflameproofed at 200° C. to 300° C. in an air atmosphere;

a carbonizing step in which the fibrous substance after theflameproofing step is carbonized at 300° C. to 2,000° C. in an inert gasatmosphere;

a first combusting step in which a carbonizing step-exhaust gasdischarged from the carbonizing step is combusted at a low oxygen ratioof 0.8 or less by a fuel and an oxygen-enriched air;

a second combusting step in which a first combusting step-exhaust gasdischarged from the first combusting step and a flameproofingstep-exhaust gas discharged from the flameproofing step are combustedusing heat of the first combusting step-exhaust gas; and

an air separating step in which nitrogen for producing the inert gasatmosphere in the carbonizing step, and the oxygen-enriched air used inthe first combusting step are produced by separating air.

Effects of Present Invention

According to the present invention, it is possible to provide an exhaustgas treatment method which prevents the generation of NO_(X), and treatsthe flameproofing step-exhaust gas and the carbonizing step-exhaust gaswith a small amount of fuel, and an exhaust gas treatment device whichcan treat the flameproofing step-exhaust gas and the carbonizingstep-exhaust gas with a small amount of fuel. In addition, according tothe present invention, it is also possible to suppress an cost increasedue to an increase in an amount of gas treated by using theoxygen-enriched air, which is produced after separating nitrogen forproducing the inert gas atmosphere in the carbonizing step, as an oxygensource for combusting the carbonizing step-exhaust gas.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view showing an exhaust gas treatment device in a carbonfiber manufacturing method of one embodiment according to the presentinvention.

FIG. 2 is a view showing another exhaust gas treatment device in acarbon fiber manufacturing method of one embodiment according to thepresent invention.

FIG. 3 is a view showing a carbon fiber manufacturing equipment of oneembodiment according to the present invention.

FIG. 4 is a graph showing a relationship between a concentration of NH₃and NO_(X) and an oxygen ratio which are discharged from the exhaust gastreatment device.

FIG. 5 is a graph showing decomposition behavior of HCN and formationand decomposition behavior of NO obtained by reaction analysis.

FIG. 6 is a graph showing decomposition behavior of NO obtained byreaction analysis when NO is added.

EMBODIMENTS OF THE INVENTION

Below, embodiments of an exhaust gas treatment method in a carbon fibermanufacturing method will be explained in detail using figures togetherwith an exhaust gas treatment device. Moreover, in order to easilyunderstand characteristics, the characteristics may be enlarged as amatter of convenience, size ratio of each components may not be the sameas that of an actual component in figures used in the followingexplanations.

First Embodiment (Exhaust Gas Treatment Device)

First embodiment of an exhaust gas treatment device according to thepresent invention is explained. FIG. 1 is a view showing a carbon fibermanufacturing equipment including an exhaust gas treatment device of thefirst embodiment according to the present invention.

As shown in FIG. 1, the exhaust gas treatment device of the firstembodiment mainly includes a first combustion furnace 10, a carbonizingstep-exhaust gas inlet 11, a first burner 30, a throttle portion 50, asecond combustion furnace 20, a flameproofing step-exhaust gas inlet 21,and an exhaust port 22.

By using the exhaust gas treatment device according to this embodiment,an exhaust gas treatment method which is explained below can be carriedout. Specifically, a carbonizing step-exhaust gas which is dischargedfrom the carbonizing furnace 2 is treated in the first combustionfurnace 10, and a flameproofing step-exhaust gas which is dischargedfrom the flameproofing furnace 1 is treated in the second combustionfurnace 20.

The first combustion furnace 10 is a tubular (for example, cylindrical)furnace which decomposes harmful gasses such as hydrogen cyanide, andammonia, contained in the carbonizing step-exhaust gas by combusting thecarbonizing step-exhaust gas. Material for the first combustion furnace10 is not particularly limited, and examples of the material include analumina refractory material, and an alumina-silica refractory material.

The first combustion furnace 10 is provided with the first burner 30, afirst thermometer (not shown in figures), and the carbonizingstep-exhaust gas inlet 11.

The first burner 30 is arranged at the end part of the first combustionfurnace 10 so as to be coaxial with the first combustion furnace 10. Thefirst burner 30 produces flam in the internal space of the firstcombustion furnace 10 by a fuel and a combustion-supporting gas. Thecarbonizing step-exhaust gas inlet 11 is provided in the side wall ofthe first combustion furnace 10 in proximity to the first burner 30. Thecarbonizing step-exhaust gas introduced into the first combustionfurnace 10 can be burned by the flame of the first burner 30.

The combustion-supporting gas supplied to the first burner 30 contains apart of oxygen-enriched air after nitrogen is separated in an airseparation device 100 for producing nitrogen to be supplied to thecarbonizing furnace 2.

By adjusting the flow rates of the fuel and the oxygen-enriched air, itis possible to control a combustion amount and an oxygen ratio, whichwill be described later. By controlling the oxygen ratio, it is possibleto form a flame in a reducing atmosphere for efficiently combusting thecarbonizing step-exhaust gas.

Material for the fuel is not particularly limited, but preferableexamples of the fuel include gas fuel, such as city gas, and LPG, andliquid fuel, such as kerosene, and a heavy oil.

From the viewpoint of improving a decomposition rate, when it is desiredto use a gas having a high oxygen concentration, the oxygenconcentration may be adjusted by adding pure oxygen in thecombustion-supporting gas conditioning equipment 101 and the gas may besupplied to the first burner 30.

When the oxygen concentration is high, the temperature in the firstcombustion furnace 10 can be raised, and the decomposition speed can beincreased. As a result, the residence time of the carbonizingstep-exhaust gas in the combustion furnace can be shortened, so that thefirst combustion furnace 10 can be reduced in size.

The exhaust gas treatment device is provided with a control unit (notshown in figures) which controls a combustion amount of the first burner30 based on the temperature in the first combustion furnace 10 and thetemperature of second combustion furnace 20.

FIG. 1 shows a case in which the carbonizing step-exhaust gas inlet 11is provided with the first combustion furnace 10. However, thecarbonizing step-exhaust gas inlet may be provided with the first burner30. Thereby, the first burner 30 produces flame in an inert atmospherein the first combustion furnace 10, and supplies the carbonizingstep-exhaust gas into the first combustion furnace 10.

The second combustion furnace 20 is provided at the post-stage of thefirst combustion furnace 10. The second combustion furnace 20 is atubular (for example, cylindrical) furnace which decomposes harmfulgasses such as hydrogen cyanide, and ammonia, contained in theflameproofing step-exhaust gas by combusting the flameproofingstep-exhaust gas. Material for the second combustion furnace 20 is notparticularly limited, but examples of the material include aluminarefractory material, and alumina-silica refractory material, similar tothe first combustion furnace 10.

The first combustion furnace 10 and the second combustion furnace 20communicate with each other via the throttle portion 50. The throttleportion 50 can prevent the combustion gas in the second combustionfurnace 20 from entering the first combustion furnace 10 and maintainthe interior of the first combustion furnace 10 in a reducingatmosphere.

The exhaust gas after combusting the carbonizing step-exhaust gas in thefirst combustion furnace 10 is supplied into the second combustionfurnace 20 via the throttle portion 50 (hereinafter, the carbonizingstep-exhaust gas after being combusted in the first combustion furnace10 is defined as “first combusting step-exhaust gas”).

In the second combustion furnace 20, the heat of the first combustingstep-exhaust gas can be used to combust the flameproofing step-exhaustgas, and harmful gas such as hydrogen cyanide contained in theflameproofing step-exhaust gas can be combusted and decomposed.

The second combustion furnace 20 is provided with a flameproofingstep-exhaust gas inlet 21 and an exhaust port 22.

The flameproofing step-exhaust gas inlet 21 is provided with the sidewall of the second combustion furnace 20 on the side of the throttleportion 50. The flameproofing step-exhaust gas can be supplied into thesecond combustion furnace 20 from the flameproofing step-exhaust gasinlet 21. The flameproofing step-exhaust gas inlet 21 is provided sothat the flameproofing step-exhaust gas can be blown in the tangentialdirection of the side wall of the second combustion furnace 20. As aresult, it is possible to form a swirling flow by the flameproofingstep-exhaust gas in the second furnace 20. The flameproofingstep-exhaust gas in the swirling flow involves the first combustingstep-exhaust gas introduced from the throttle portion 50, andefficiently combusts and decomposes the harmful gas contained in theflameproofing step-exhaust gas.

Moreover, as shown in FIG. 2, the second burner 31 may be provided at aposition on the side wall of the second combustion furnace 20 oppositeto the flameproofing step-exhaust gas inlet 21. It is possible to finelyadjust the temperature in the second combustion furnace 20 by supplyingthe fuel to the second burner.

It is possible to discharge the gas combusted in the second combustionfurnace 20 from the exhaust port 22 provided at the end wall of thesecond combustion furnace 20 which is opposite to the throttle portion50.

(Exhaust Gas Treatment Method)

Below, the exhaust gas treatment method in the carbon fibermanufacturing equipment using the exhaust gas treatment device will beexplained.

In the carbon fiber manufacturing system in the present embodimentincludes a flameproofing step in which the fibrous substance issubjected to a flameproofing treatment at 200 to 300° C. in an airatmosphere in the flameproofing furnace 1, and a carbonizing step inwhich a carbonizing treatment is carried out at 300 to 2,000° C. in aninert gas atmosphere in the carbonizing furnace 2 provided at thepost-stage of the flameproofing furnace 1.

In order to be an inert gas atmosphere in the carbonizing furnace 2,nitrogen is separated from air using the air separation device 100, andthe nitrogen is supplied into the carbonizing furnace 2.

The carbonizing step-exhaust gas generated in the carbonizing furnace 2is subjected to a first combusting step in the first combustion furnace10. The flameproofing step-exhaust gas generated in the flameproofingfurnace 1 is subjected to a second combusting step in the secondcombustion furnace 20.

The first combusting step is a combustion step in which the carbonizingstep-exhaust gas is combusted at a low oxygen ratio, an oxygen ratio of0.8 or less. Specifically, in the first combusting step, the exhaust gas(carbonizing step-exhaust gas) which is generated in the carbonizingstep in the carbonizing furnace 2 and based on nitrogen gas containinghydrogen cyanide, ammonia, etc. at high concentrations is supplied intothe first combustion furnace 10 through the carbonizing step-exhaust gasinlet 11. A fuel and a combustion-supporting gas are combusted in atemperature range from 1,000 to 1,600° C. by the first burner 30provided in the first combustion furnace 10, and a flame is produced.

The combustion-supporting gas supplied to the first burner 30 is anoxygen-enriched air after separating nitrogen from raw air in the airseparation device 100. The carbonizing step-exhaust gas is combusted inthe first combustion furnace 10 by the flame produced by the firstburner 30 using the oxygen-enriched air and the fuel.

At this time, the carbonizing step-exhaust gas may be supplied from thefirst burner 30 into the first combustion furnace 10.

The temperature inside of the first combustion furnace 10 is measured bythe first thermometer (not shown in figures). The temperature inside ofthe second combustion furnace 20 is measured by a second thermometer(not shown in figures). Based on the temperatures measured, the controlunit (not shown in figures) adjusts the combustion temperature bycontrolling the combustion amount of the first burner 30. The combustionamount can be controlled by adjusting the amount of the fuel gas and thecombustion-supporting gas supplied.

Moreover, “combustion amount” is an amount of heat generated per unittime caused by combusting a fuel. As the combustion amount increases,the amount of heat generated per unit time increases, so that thetemperature of the furnace increases.

Since the carbonizing step-exhaust gas to be treated in the firstcombustion furnace 10 is a nitrogen-based exhaust gas containinghydrogen cyanide, ammonia, and the like at a high concentration, whenthe combustion treatment is carried out under conditions in which theoxygen ration is higher than the stoichiometric ratio (oxygen ratio ishigher than 0.8), a large amount of NO_(X) is produced. Therefore, inthe first combustion furnace 10, treatment is carried out while forminga reducing atmosphere under combustion conditions with an oxygen ratioof 0.8 or less.

Thereby, it is possible to carry out combustion and decomposition whilesuppressing the generation of NO_(x). Therefore, in the exhaust gastreatment method of this embodiment, the oxygen ratio is controlled to0.8 or less by decreasing the ratio of oxygen contained in thecombustion-supporting gas with respect to the fuel gas.

Moreover, the “oxygen ratio” is a value obtained by dividing the oxygenamount supplied to the burner by the theoretical oxygen amount requiredfor combusting the fuel supplied to the burner. Therefore,theoretically, it can be said that the state where the oxygen ratio is1.0 is a state in which complete combustion can be carried out usingexcessive or insufficient oxygen.

As explained above, the harmful gas such as hydrogen cyanide, ammoniaand the like contained in the carbonizing step-exhaust gas is combustedand decomposed in the first combustion furnace 10. The first combustingstep-exhaust gas generated by combustion is supplied into the secondcombustion furnace 20 via the throttle portion 50.

The second combusting step is a combustion step in which the firstcombustion step-exhaust gas discharged in the first combusting step andthe flameproofing step-exhaust gas are combusted using the heat of thefirst combustion step-exhaust gas.

Incidentally, the flameproofing step-exhaust gas is an air-based exhaustgas containing hydrogen cyanide and ammonia, and has a much largerdischarge amount than that of the carbonizing step-exhaust gas.Therefore, when the oxygen ratio is reduced to 0.8 or less and thecombustion decomposition is attempted in the same way as the carbonizingstep-exhaust gas, it is necessary to use a large amount of fuel.

In addition, hydrogen cyanide and ammonia can be decomposed whilesuppressing generation of NO_(X) by performing combustion treatment at alow temperature even in an atmosphere in which oxygen exists.

Therefore, in the exhaust gas treatment method of the presentembodiment, hydrogen cyanide and ammonia are decomposed whilesuppressing generation of NO_(X) by combusting the flameproofingstep-exhaust gas in the temperature range of 700 to 1,200° C.

Firstly, the first combusting step-exhaust gas supplied from the firstcombustion furnace 10 through the throttle portion 50 is mixed with theflameproofing step-exhaust gas supplied from the flameproofingstep-exhaust gas inlet 21 provided in the second combustion furnace 20.For example, the flameproofing step-exhaust gas is introduced from theside wall of the cylindrical second combustion furnace 20 in thetangential direction of the side wall. The flameproofing step-exhaustgas introduced in this way forms a swirling flow while involving thefirst combusting step-exhaust gas. Thereby, the first combustingstep-exhaust gas and the flameproofing step-exhaust gas can besufficiently mixed. In addition, the residence time in the secondcombustion furnace can be increased by making the swirling flow with theflameproofing step-exhaust gas.

When flameproofing step-exhaust gas and the first combustingstep-exhaust gas are mixed in the second combustion furnace 20, the gassuch as CO or H₂ contained in the first combusting step-exhaust gas andthe oxygen contained in the flameproofing step-exhaust gas arecombusted. By both the heat generated by the combustion and the heat ofthe first combusting step-exhaust gas, the temperature inside the secondcombustion furnace 20 can be raised to 700° C. or higher. When thetemperature in the second combustion furnace 20 reaches 700° C. orhigher, the harmful gas such as hydrogen cyanide contained in theflameproofing step-exhaust gas is combusted and decomposed. Thus, in thesecond combusting step, the heat of the first combusting step-exhaustgas discharged in the first combusting step is effectively utilized.

When the amount of heat for effectively carried out the secondcombusting step is insufficient, for example, it is possible to providea second burner 31 to the second combustion furnace 20, supply the fuel,and increase the temperature.

In addition, it is also possible to stably combust the flameproofingstep-exhaust gas and the first combusting step-exhaust gas by providingthe second burner 31 to the inside wall of the second combustion furnace30. The combustion amount can be controlled by supplying the fuel to thesecond burner 31 and adjusting the flow rate of the fuel supplied to thesecond burner 31. It is also possible to control the combustionconditions by supplying the fuel and the combustion-supporting gas tothe second burner 31. The second burner 31 does not need to beconstantly burned, and it may be ignited when the internal temperatureof the second combustion furnace 20 becomes lower than the predeterminedtemperature.

The temperature inside the second combustion furnace 20 is measured bythe second thermometer (not shown in figures). By controlling the oxygenratio of the first burner 30 by the control unit (not shown in figures)based on the measured temperature, the amount of uncombusted gas flowinginto the second combustion furnace 20 is controlled. Thus, thetemperature in the second combustion furnace 20 can be controlled.

Next, the exhaust gas generated by combustion in the second combustionfurnace 20 is exhausted to the outside from the exhaust port 22, therebycompleting the exhaust gas treatment method of the present embodiment.

As described above, the carbon fiber manufacturing equipment includingthe exhaust gas treatment device of the present embodiment includes theflameproofing furnace 1 for subjecting the fibrous substance to theflameproofing treatment in an air atmosphere, the carbonizing furnace 2for subjecting the fibrous substance after the flameproofing treatmentto the carbonizing treatment in an inert gas atmosphere, the airseparation device 100 for separating nitrogen, which is supplied intothe carbonizing furnace 2, from air, the first combustion furnace 10 fortreating the carbonizing step-exhaust gas generated in the carbonizingfurnace 2, the first burner 30 which is provided in the first combustionfurnace 10 and combusts the oxygen-enriched air from the air separationdevice 100 and the fuel, and the second combustion furnace 20 fortreating the flameproofing step-exhaust gas generated in theflameproofing furnace, and the second combustion furnace 20 is providedat the post-stage of the first combustion furnace 10, the firstcombustion furnace 10 and the second combustion furnace 20 communicatewith each other via the throttle portion 50, the first combustingstep-exhaust gas after combustion in the first combustion furnace 10 canbe supplied into the first combustion furnace 20. The flameproofingstep-exhaust gas inlet 21 is provided in the second combustion furnace20 so that the flameproofing step-exhaust gas can be blown in thetangential direction of the side wall of the second combustion furnace20. The flameproofing step-exhaust gas forms the swirling flow in thesecond combustion furnace 20. Thereby, the flameproofing step-exhaustgas is combusted by mixing with the first combusting step-exhaust gassupplied from the throttle portion 50 into the second combustionfurnace.

In the second combustion furnace 20, since the heat of the firstcombusting step-exhaust gas can be used, the flameproofing step-exhaustgas can be efficiently combusted. As a result, the amount of the fuelused to treat the carbonizing step-exhaust gas and the flameproofingstep-exhaust gas can be reduced. In addition, since the carbonizingstep-exhaust gas and the flameproofing step-exhaust gas can be treatedby one device, the equipment cost and the maintenance cost can bereduced.

In addition, since the second combustion furnace 20 is communicated withthe first combustion furnace 10 through the throttle portion 50 in theexhaust gas treatment device 1 of the present embodiment, it is possibleto prevent the gas including oxygen in the second combustion furnace 20from entering into the first combustion furnace 10, and maintain theinside of the first combustion furnace 10 in the reducing atmosphere.

In addition, according to the exhaust gas treatment method of thisembodiment, since the nitrogen gas which is used in the carbonizing stepin an inert gas atmosphere is produced by the air separation device 100,the remaining oxygen-enriched air after separation of nitrogen from theair is used as the combustion-supporting gas in the first combustingstep, it is possible to suppress the cost increase due to upsizing ofthe carbon fiber manufacturing equipment.

In addition, since the carbonizing step-exhaust gas is combusted at alow oxygen ratio having an oxygen ratio of 0.8 or less in the firstcombusting step, the carbonizing step-exhaust gas can be treated whilesuppressing generation of NOx.

Further, according to the exhaust gas treatment method of thisembodiment, since the second combusting step in which the firstcombustion step-exhaust gas and the flameproofing step-exhaust gas arecombusted is carried out by using the heat of the first combustingstep-exhaust gas discharged from the first combusting step in which thecarbonizing step-exhaust gas is treated, the amount of the fuel used inthe burner can be reduced. In addition, since the carbonizingstep-exhaust gas and the flameproofing step-exhaust gas can be treatedin continuous steps, the equipment cost and the maintenance cost can bereduced.

Second Embodiment

Next, a carbon fiber manufacturing equipment of a second embodimentaccording to the present invention will be described. FIG. 3 is a viewshowing a carbon fiber manufacturing equipment of a second embodimentaccording to the present invention. Here, a description will be givenfocusing on parts different from the first embodiment.

As shown in FIG. 3, the carbon fiber manufacturing equipment of thisembodiment is different from the first embodiment in that thegraphitizing furnace 3 is provided at the post-stage of thecarbonization furnace 2.

In the graphitizing furnace 3, carbon fiber having high elasticity canbe obtained by heating the carbonized fibrous substance in an inert gasatmosphere at 2,000° C. to 3,000° C. As in the carbonizing treatment,the graphitizing treatment is performed in an inert gas atmosphere, sothat a part of the nitrogen produced by the air separation device 100 isalso introduced into the graphitizing furnace 3.

A graphitizing step-exhaust gas is a nitrogen-based gas containinghydrogen cyanide, ammonia and the like at high concentrations.Therefore, the graphitizing step-exhaust gas can be treated in the firstcombustion furnace 10 together with the carbonizing step-exhaust gas.The graphitizing step-exhaust gas may be introduced into the firstcombustion furnace 10 after mixing with the carbonizing step-exhaustgas, or may be separately introduced. FIG. 3 shows a case in which thesegases are introduced from the graphitizing step-exhaust gas inlet 11after mixing the graphitizing step-exhaust gas and the carbonizingstep-exhaust gas in a buffer tank (not shown) or the like.

The technical scope of the present invention is not limited to the aboveembodiments, and various modifications can be made without departingfrom the spirit of the present invention.

For example, in the exhaust gas treatment device of the above-describedembodiment, the exhaust gas combusted in the second combustion furnace20 is discharged to the outside through the exhaust port 22. However, aheat exchanger may be connected to the exhaust port 22, the exhaust gasintroduced into each combustion furnace may be preheated by utilizingthe heat of the post-treatment exhaust gas discharged from the exhaustport 22. Thereby, it possible to lower the combustion amount of theburner of each combustion furnace, and reduce the amount of fuel used.

Example 1

(Comparison with Direct-Combustion Method)

Using the exhaust gas treatment device shown in FIG. 1 and aconventional direct-combustion type exhaust gas treatment device of theprior art, treatment tests were carried out using simulated gas of thecarbonizing step-exhaust gas discharged from the carbonizing furnace,the graphitizing step-exhaust gas discharged from the graphitizingfurnace, and the flameproofing step-exhaust gas discharged from theflameproofing furnace.

Table 1 shows the composition and flow rate of the simulated gas (here,it is denoted by simulated gas A) of the carbonizing step-exhaust gasand the graphitizing step-exhaust gas, and the simulated gas (here, itis denoted by simulated gas B) of the flameproofing step-exhaust gas.For simulated gas, NO was used as an alternative to HCN (the validity ofusing NO as a simulated gas will be described later). In this treatmenttest, simulated gas was treated under three conditions (Conditions 1-1,1-2, and 1-3).

Table 2 shows the combustion conditions of the burner of the exhaust gastreatment device and the direct-combustion type exhaust gas treatmentdevice as a comparative example.

In this example, oxygen-enriched air having an oxygen concentration of40% was used as the combustion-supporting gas and combusted at an oxygenratio of 0.7 in the first burner 30. The temperature of the firstcombustion furnace 10 was 1,600° C. and the temperature of the secondcombustion furnace 20 was 1000° C.

In the direct-combusting type treatment device, treatment was carriedout at 1,000° C.

TABLE 1 Condition Condition Condition 1-1 1-2 1-3 Simulated NOconcentration 5 2.5 1 gas A [vol %] NH₃ concentration 5 2.5 1 [vol %]Base gas Nitrogen Nitrogen Nitrogen Flow rate [Nm³/h] 1 1 1 Simulated NOconcentration 0 0 gas B [vol %] NH₃ concentration 0.01 0.01 0.01 [vol %]Base gas Air Air Air Flow rate [Nm³/h] 10 10 10

TABLE 2 Exhaust Direct- gas treatment combustion device type exhaustshown in gas treatment FIG 1 device Flow rate of city gas [Nm³/h] 1.3 3Flow rate of combustion-supporting 5.3 — gas [Nm³/h] Flow rate of air[Nm³/h] — 33 Oxygen concentration of 40 20.8 combustion-supporting gas[vol %]

Table 3 shows the treatment test results. In the table, the results ofthe exhaust gas treatment device shown in FIG. 1 are represented asInventions 1 to 3, and the results of the direct-fire type exhaust gastreatment device are shown as Comparative Examples 1 to 3.

From these results, it was confirmed that ammonia (NH₃) is decomposed toan extremely low concentration, and the generation of NOx is suppressedto about 90 ppm in the exhaust gas treatment device shown in FIG. 1 evenunder the condition 1-1 in which NO and NH₃ are added at the highestconcentration.

On the other hand, it was confirmed that when NO and NH₃ are decomposed,the NO_(X) concentration increases in the direct-combustion type exhaustgas treatment system.

Further, it was also confirmed that the carbonizing step-exhaust gas,the graphitizing step-exhaust gas, and the flameproofing step-exhaustgas are treated with fewer fuel in the exhaust gas treatment deviceshown in FIG. 1 than the direct-combustion type exhaust gas treatmentdevice.

TABLE 3 Condition 1-1 Condition 1-2 Condition 1-3 Present ComparativePresent Comparative Present Comparative Invention 1 Example 1 Invention2 Example 2 Invention 3 Example 3 NH₃ 0 5 0 3 0 0 concentration [ppm]NOx 90 1210 75 730 58 350 concentration [ppm]

Example 2 (Effect of Oxygen Ratio)

Using the same exhaust gas treatment device shown in FIG. 1 similar toExample 1, the concentration of NH₃ and NOx contained in the exhaust gasafter treating the simulated gas A and the simulated gas B under thecondition 1-2 of Table 3 were measured by changing the oxygen ratio ofthe first burner 30 as shown in Table 4.

TABLE 4 Condition Condition Condition Condition Condition Condition 2-12-2 2-3 2-4 2-5 2-6 Flow rate of city gas 1.3 1.3 1.3 1.3 1.3 1.3[Nm³/h] Flow rate of 1.8 2.1 2.4 2.7 3.0 3.3 combustion-supporting gas[Nm³/h] Oxygen ratio [—] 0.6 0.7 0.8 0.9 1.0 1.1 Oxygen concentration 3030 30 30 30 30 of combustion-supporting gas [vol %]

FIG. 4 shows the relationship between the concentration of NH₃ and NOxin the exhaust gas after treating discharged from the exhaust port 22 ofthe exhaust gas treatment device and the oxygen ratio.

From these results, it was confirmed that NH₃ is less than 0.1 ppm underall conditions, and almost all can be decomposed.

Further, when the oxygen ratio of the first burner 30 is set to belarger than 0.8, the NOx tends to rapidly increase. It was confirmedthat the simulated gas A is treated while suppressing the generation ofNOx by setting the oxygen ratio to 0.8 or less.

Example 3 (Test on Pilot Equipment)

Using the exhaust gas treatment device shown in FIG. 1 as a pilotequipment, exhaust gas treatment was performed.

Table 5 shows the composition and flow rate of the simulated gas A ofthe carbonizing step-exhaust gas and the graphitizing step-exhaust gas,and the simulated gas B of the flameproofing step-exhaust gas. The flowrate of the simulated gas B was adjusted to 300, 600, and 900 Nm³/h(conditions 3-1, 3-2, and 3-3). Table 6 also shows the burner combustionconditions under each exhaust gas condition.

TABLE 5 Condition Condition Condition 3-1 3-2 3-3 Simulated NOconcentration 5 5 5 gas A [vol %] NH₃ concentration 5 5 5 [vol %] Basegas Nitrogen Nitrogen Nitrogen Flow rate [Nm³/h] 30 30 30 Simulated NOconcentration 0 0 0 gas B [vol %]0 NH₃ concentration 0.01 0.01 0.01 [vol%] Base gas Air Air Air Flow rate [Nm³/h] 300 600 900

TABLE 6 Condition Condition Condition 3-1 3-2 3-3 Flow rate of city gas[Nm³/h] 15 23 31 Flow rate of combustion-supporting 10 15 20 gas [Nm³/h]Flow rate of air [Nm³/h] 73 112 150 Oxygen concentration of combustion-30 30 30 supporting gas [vol %]

Table 7 shows the concentrations of NH₃ and NO x in the exhaust gasafter treating discharged from the exhaust port 22 of the exhaust gastreatment device. From these results, it was confirmed that the exhaustgas treatment device shown in FIG. 1 decomposes NH₃ to an extremely lowconcentration, and furthermore, it is possible to suppress generation ofNOx accompanying combustion.

TABLE 7 Condition 3-1 Condition 3-2 Condition 3-3 NH₃ concentration 0.80.6 0.5 [ppm] NOx concentration 55 38 25 [ppm]

Example 4 (Validation of Using Simulated Gas)

NO was used as an alternative simulated gas for HCN. The validity ofusing NO as simulated gas was studied by reaction analysis in thesimulation.

Reaction analysis was performed using CHEMKIN-PRO (Reaction Design,detailed chemical reaction analysis support software). The analysisconditions are shown in Table 8. Condition 4-1 is a case where HCN isadded to the first combustion furnace 10 under a reducing atmosphere andcondition 4-2 is a case where NO is added.

TABLE 8 Condition 4-1 Condition 4-2 Fuel CH₄ [mol] 1.0 1.0 Oxygen O₂[mol] 1.6 1.6 Gas to be treated HCN 0.1 — [mol] NO — 0.1 N₂ 0.9 0.9Reaction temperature [° C.] 1500 1500

FIG. 5 shows the decomposition behavior of HCN and thegeneration/decomposition behavior of NO by the reaction analysis underCondition 4-1. In addition, FIG. 6 shows the NO decomposition behaviorby the reaction analysis when NO is added under Condition 4-2.

It is understood from FIG. 5 that HCN is rapidly decomposed in areducing combustion atmosphere, and NOx is rapidly generated due to thedecomposition of HCN, and then NOx generated is gradually decomposed.Comparing the change in NO concentration in FIGS. 5 and 6, thedecomposition behavior shows the same trend, and it is possible toevaluate the decomposition behavior of NO generated along withdecomposition of HCN by using NO as the simulation gas.

Example 5

In the carbon fiber manufacturing equipment shown in FIG. 2, nitrogenwhich was separated from a raw air by the air separation device isintroduced into the carbonizing furnace and the remaining oxygenenriched-air after separation of the nitrogen from the raw air was usedas the combustion-supporting gas for combusting the carbonizingstep-exhaust gas.

TABLE 9 Second Air separation Carbonizing Flameproofing First combustioncombustion device furnace furnace furnace furnace Flow rate of raw air11,300 to — — — — [Nm³/h] 34,000 Flow rate of nitrogen introduced into5,000 to — — — — carbonizing furnace 15,000 [Nm³/h] Flow rate ofcarbonizing step-exhaust gas — 5,000 to 15,000 [Nm³/h] Flow rate offlameproofing step-exhaust gas — 15,000 to 410,000 [Nm³/h] Concentrationof oxygen in oxygen-enriched — 40 air introduced into first combustionfurnace [vol %] Flor rate of oxygen-enriched air introduced — 1,100 to3,300 into first combustion furnace [Nm³/h] Flow rate of city gasintroduced into first — 108 to 300 burner [Nm³/h] Flow rate of city gasintroduced into second — 0 to 3,800 burner [Nm³/h]

In this example, city gas and oxygen-enriched air having an oxygenconcentration of 40% were combusted at an oxygen ratio of 0.65 in thefirst burner 30. The temperature of the first combustion furnace 10 was1,600° C. and the temperature of the second combustion furnace 20 was1000° C.

Example 6

In the carbon fiber manufacturing equipment shown in FIG. 3, nitrogenwhich was separated from a raw air by the air separation device isintroduced into the carbonizing furnace and the graphitizing furnace,and the remaining oxygen enriched-air after separation of the nitrogenfrom the raw air was used as the combustion-supporting gas forcombusting the carbonizing step-exhaust gas and graphitizingstep-exhaust gas.

TABLE 10 Second Air separation Carbonizing Flameproofing Firstcombustion combustion device furnace furnace furnace furnace Flow rateof raw air 8,800 to 11,400 — — — — [Nm³/h] Flow rate of nitrogenintroduced into 3,000 to 5,000  — — — — carbonizing furnace andgraphitizing furnace [Nm³/h] Flow rate of carbonizing step-exhaust gasand — 3,000 to 5,000 graphitizing step-exhaust gas [Nm³/h] Flow rate offlameproofing step-exhaust gas — 9,500 to 270,000 [Nm³/h] Concentrationof oxygen in oxygen-enriched — 40 air introduced into first combustionfurnace [vol %] Flor rate of oxygen-enriched air introduced —   640 to1,050 into first combustion furnace [Nm³/h] Flow rate of city gasintroduced into first — 45 to 75 burner [Nm³/h] Flow rate of city gasintroduced into second — 32 to 1,750 burner [Nm³/h]

INDUSTRIAL APPLICABILITY

The exhaust gas treatment method and the exhaust gas treatment device ofthe present invention can be used as a method and a device for treatingexhaust gas containing hydrogen cyanide, ammonia and the like.

In particular, it is possible to suppress the generation of NOx andtreat the flameproofing step-exhaust gas and the carbonizingstep-exhaust gas in carbon fiber manufacturing with a small amount offuel, and suppress the cost increase due to an increase in an amount ofgas treated.

DESCRIPTION OF REFERENCE NUMERALS

-   1 flameproofing furnace-   2 carbonizing furnace-   3 graphitizing furnace-   10 first combustion furnace-   11 carbonizing step-exhaust gas inlet-   20 second combustion furnace-   21 flameproofing step-exhaust gas inlet-   22 exhaust port-   30 first burner-   31 second burner-   50 throttle portion-   100 air separation device-   101 combustion-supporting gas conditioning equipment

1. An exhaust gas treatment device which is configured to treat exhaust gases discharged from steps of manufacturing carbon fiber from a fibrous substance, wherein the exhaust gas treatment device includes: a first combustion furnace which is configured to combust a carbonizing step-exhaust gas discharged from a carbonizing step in which the fibrous substance is carbonized in an inert gas atmosphere; and a second combustion furnace which is provided at a post-stage of the first combustion furnace, and configured to combust a flameproofing step-exhaust gas discharged from a flameproofing step in which the fibrous substance is flameproofed in an air atmosphere and a first combusting step-exhaust gas discharged from the first combustion furnace; the first combustion furnace and the second combustion furnace communicate through a throttle portion; the first combustion furnace is provided with a first burner which is configured to combust a carbonizing step-exhaust gas by a fuel and an oxygen-enriched air; and an air separation device which is configured to produce nitrogen for producing the inert gas atmosphere in the carbonizing step and the oxygen-enriched air supplied into the first burner.
 2. The exhaust gas treatment device according to claim 2, wherein the exhaust gas treatment device further includes a second burner which is configured to supply a fuel into the second combustion furnace.
 3. An exhaust gas treatment method for treating exhaust gases discharged from steps of manufacturing carbon fiber from a fibrous substance, wherein the exhaust gas treatment method includes: a first combusting step in which a carbonizing step-exhaust gas discharged from a carbonizing step in which the fibrous substance is carbonized in an inert gas atmosphere is treated; and a second combusting step in which a flameproofing step-exhaust gas discharged from a flameproofing step in which the fibrous substance is flameproofed in an air atmosphere and a first combusting step-exhaust gas discharged from the first combustion step are treated; in the first combusting step, the carbonizing step-exhaust gas is combusted at a low oxygen ratio of 0.8 or less by a fuel and an oxygen-enriched air; in the second combusting step, the first combusting step-exhaust gas and the flameproofing step-exhaust gas are combusted by using heat of the first combusting step-exhaust gas; and nitrogen for producing the inert gas atmosphere in the carbonizing step, and the oxygen-enriched air used in the first combusting step are produced by separating air.
 4. The exhaust gas treatment method according to claim 3, wherein the combustion of the first combusting step-exhaust gas and the flameproofing step-exhaust gas is promoted by supplying a fuel in the second combusting step.
 5. A carbon fiber manufacturing equipment including equipment which is configured to produce carbon fiber from a fibrous substance, and exhaust gas treatment equipment which is configured to treat exhaust gases from the equipment which is configured to produce carbon fiber from a fibrous substance, wherein the carbon fiber manufacturing equipment includes: a flameproofing furnace which is configured to pre-oxidize and flameproof the fibrous substance in an air atmosphere; a carbonizing furnace which is provided at a post-stage of the flameproofing furnace and configured to carbonize the fibrous substance after flameproofing in an inert atmosphere; a first combustion furnace which is configured to combust a carbonizing step-exhaust gas discharged from the carbonizing furnace; a second combustion furnace which is configured to communicate with the first combustion furnace through a throttle portion, and combust a first combusting step-exhaust gas and a flameproofing step-exhaust gas discharged from the flameproofing furnace; a first burner which is provided in the first combustion furnace so as to combust the carbonizing step-exhaust gas by a fuel and an oxygen-enriched air; and an air separation device which is configured to produce nitrogen for producing the inert gas atmosphere in the carbonizing furnace and the oxygen-enriched air supplied into the first burner from air.
 6. A carbon fiber manufacturing system including a step in which carbon fiber is produced from a fibrous substance and a step in which exhaust gases discharged from the step in which carbon fiber is produced from a fibrous substance are treated, wherein the carbon fiber manufacturing system includes: a flameproofing step in which the fibrous substance is pre-oxidized and flameproofed at 200° C. to 300° C. in an air atmosphere; a carbonizing step in which the fibrous substance after the flameproofing step is carbonized at 300° C. to 2,000° C. in an inert gas atmosphere; a first combusting step in which a carbonizing step-exhaust gas discharged from the carbonizing step is combusted at a low oxygen ratio of 0.8 or less by a fuel and an oxygen-enriched air; a second combusting step in which a first combusting step-exhaust gas discharged from the first combusting step and a flameproofing step-exhaust gas discharged from the flameproofing step are combusted using heat of the first combusting step-exhaust gas; and an air separating step in which nitrogen for producing the inert gas atmosphere in the carbonizing step, and the oxygen-enriched air used in the first combusting step are produced by separating air. 